[codex] Prehash resolver path dependencies

[hardfist]

Summary

• Add PathDependency and ResolvePreHashedContext so callers can consume resolver dependency paths with their precomputed path hash.
• Reuse the same path hashing helper for the internal path cache and expose the identity hasher needed by the prehashed dependency set.
• Short-circuit cached path equality by checking the precomputed hash before comparing full paths.

Why

Rspack build_module_graph path dependency handling currently hashes dependency paths in the resolver context and then hashes the same paths again when converting them into Rspack prehashed ArcPath sets. This API lets Rspack reuse the resolver-side hash instead of paying for another full Path hash during that conversion.

Validation

• cargo check
• cargo test tests::dependencies --lib
• Full cargo test was also attempted; 124 tests passed and 6 existing PnP fixture tests failed with NotFound(...), which appears unrelated to this change.

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing codex/prehash-path-dependencies (1bef3bc) with main (31610e6)}